This invention relates to a method of roughening solid surfaces, that is, creating a multiplicity of closely spaced pits and projections on or in such surfaces. The invention lends itself to the surface roughening of light-emitting semiconductor devices, or light-emitting diodes (LEDs) in more common parlance, for reduction of internal total reflection of the light, and of semiconductor substrates for epitaxial growth of low-dislocation-density layers thereon, among other applications.
An LED may be envisaged for the purposes of the invention as a combination of a light-generating chip of semiconducting material and a protective envelope of transparent plastic material therefor. The semiconductor chip of familiar design has a set of layers for generating light, and a supplementary layer (termed variously as a current-spreading layer, contact layer, etc.) that has a surface from which there is emitted the light radiating from the light-generating layers. Issuing from the surface of the supplementary layer, the light has to traverse the protective envelope.
As is well known, total internal reflection occurs at the boundary surface between two dissimilar materials when the light strikes the surface at an angle greater than the critical angle which is determined by the refractive indices of the materials in question. The noted supplementary layer of the semiconductor chip has a refractive index of 3.1-3.5 whereas the protective envelope has that of 1.5 or so. The critical angle in this case is 25-29 degrees. Those of the light rays radiated in various directions in the semiconductor chip which impinge on the boundary surface at angles greater than the critical angle with respect to the normal to the surface are totally reflected away therefrom, with a consequent drop in the efficiency of the LED.
A remedy, itself well known in the art, to the undesired total internal reflection of LEDs is the roughening of the light-emitting surface of the chip, as disclosed for example in Japanese Unexamined Patent Publication No. 2003-209283. It teaches to roughen the chip surface by blade machining or by photolithography using a photomask and photoresist. Blade machining is objectionable by reason of tools and associated equipment required. Photolithography on the other hand demands the fabrication of a finely patterned photomask, and it is extremely difficult, and costly too, to unerringly create surface unevenness in the order of tens or hundreds of nanometers.
Another possible application of the invention may now be briefly explained by way of reconfirmation of the utility of the surface-roughening method according to the invention. The method lends itself to use in the fabrication of dislocation-free semiconductor layers by what is known as microchannel epitaxy (MCE). The MCE is such that a semiconductor crystal is grown by epitaxy on a semiconductor surface which has been previously roughened with pits. The crystal grows not only upwardly but laterally of the semiconductor surface, totally filling the pits in the semiconductor surface. There is thus obtained a flat-surfaced, dislocation-free crystal layer. The roughening of the semiconductor surface preliminary to MCE has encountered the same difficulties as with LEDs discussed above.